Semiconductor-on-insulator (SOI) devices are widely used in microelectronics. In general, SOI devices include active devices such as transistors in a thin semiconductor layer which is on an insulator. In contrast, bulk semiconductor devices include active devices such as transistors in a bulk semiconductor region. SOI devices often use a layer of monocrystalline silicon as a semiconductor material. These devices are often referred to as silicon-on-insulator devices.
Referring to FIGS. 1A-1E, a conventional fabrication method for an SOI device will be described. As shown in FIG. 1A, N- or P-type dopants are implanted and diffused into a first substrate 1, at a high doping concentration, to form a buried layer 3. As shown in FIG. 1B, an insulating layer 5 such as an oxide film, is formed on the first substrate 1. As shown in FIG. 1C, a second substrate 7 which is generally not doped, is bonded on the oxide film 5.
As shown in FIG. 1D, the first substrate 1 is ground, etched or polished to a predetermined thickness. As shown in FIG. 1E, an oxide film is formed on the polished surface of the first substrate 1 and patterned to form a mask. The first substrate 1 and the buried layer 3 are etched, using the oxide film as a mask, to form a trench 9. The trench preferably extends to the insulating layer 5. Regions between the trenches are active regions A, where active devices such as transistors are formed. An oxide film 11 is then formed in the trenches 9, for example by thermal oxidation.
Unfortunately, microelectronic circuits often employ transistors of opposite conductivity types which may be desirably formed in active layers of different conductivity types. It may be difficult to vary the conductivity type in different regions of the buried layer 3. Moreover, it ions are selectively implanted, or the insulator 5 is selectively etched, it may difficult to align elements to these regions in subsequent processing steps.